1. Field of the Invention
The present invention generally relates to a method and apparatus for transmitting/receiving paging information in a wireless communication system. More particularly, the present invention relates to a method and apparatus for transmitting/receiving paging information in a cellular system supporting scalable bandwidth.
2. Description of the Related Art
Universal Mobile Telecommunication System (UMTS) is a 3rd Generation (3G) asynchronous Wideband Code Division Multiple Access (WCDMA) mobile communication system based on European mobile communication systems, Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS).
The 3rd Generation Partnership Project (3GPP) group working on UMTS standardization is considering Long Term Evolution (LTE) for future-generation mobile communications. LTE is a technology for realizing high-speed packet communications at or above 100 Mbps (Megabits per second). For radio access in an LTE system, Orthogonal Frequency Division Multiplexing (OFDM) is under active discussion as promising for high-speed data transmission.
Due to orthogonality between subcarriers, OFDM offers optimal transmission efficiency in high-speed data transmission. Also, overlapped frequency spectrum leads to good frequency use efficiency and robustness against multipath fading contributes to the optimal transmission efficiency of high-speed data transmission.
One significant feature of an OFDM cellular wireless communication system that provides high-speed data service is to support scalable bandwidth. A bandwidth-scalable system can have a variety of frequency bandwidths, for example, 20, 15, 10, 5, 2.5, and 1.25 MHz (MegaHertz). Service providers can select one of the bandwidths for each cell to provide services, and there may exist User Equipments (UEs) that support different bandwidths ranging from 1.25 MHz to 20 MHz.
In a bandwidth-scalable system, a UE should succeed in a cell search without knowledge of a system bandwidth during an initial access to the system. The UE acquires synchronization between a transmitter and a receiver and a cell identifier (ID) by a cell search in order to demodulate data and control information. The system bandwidth is known from a Synchronization CHannel (SCH) signal during the cell search or by demodulating a Broadcasting CHannel (BCH) that is a shared control channel carrying system information about the cell after the cell search. The BCH is the first channel that the MS demodulates after the cell search. The MS performs the cell search by the SCH signal and acquires the system information about the cell by receiving the BCH signal after a successful cell search in the cell. The system information is used for receiving data channels and control channels, such as the cell ID, the system bandwidth, channel setup information, etc.
The cellular wireless communication system uses a Paging CHannel (PCH) to page an idle-state UE and establish a connection to the UE. In the presence of paging information for the UE in the PCH signal, the UE starts a connection setup procedure with the system. The PCH signal can include Paging Indicators (PIs) and paging messages. Each PI is composed of a group ID identifying a group to which the UE belongs and scheduling information indicating resources in which a paging message directed to the UE is transmitted. The group ID is a representative of the IDs of a plurality of UEs. The paging message includes the ID of the UE to be paged within the group indicated by the PI and information indicating the purpose of paging. The idle-state UE wakes up every period to monitor the presence or absence of transmission information from a Node B (or Base Station (BS)) in order to save power. This is called Discontinuous Reception (DRX). That is, the idle-state UE wakes up every DRX period, receives a PCH signal, and detects a group ID in a PI. The UE receives a paging message when the group ID is identical to that of the group to which the UE belongs. When the group IDs are different, the UE is kept in the idle state until the next DRX period. When a UE ID included in the paging message is identical to that of the UE, the UE detects the purpose of the paging and performs a reception operation corresponding to the system paging. When the paging message does not include the ID of the UE, the UE is kept in the idle state until the next DRX period. This paging procedure and DRX operation reduces unnecessary power consumption of the UE.
The bandwidth-scalable system differs from a legacy system in that UEs have different bandwidth capabilities. Therefore, a significant task that the bandwidth-scalable system has to deal with in using the PCH is that a UE having a less bandwidth than a system bandwidth should be able to receive the PCH signal reliably even when the UE receives a service in part of the system bandwidth.
FIG. 1 a conventional example of idle-state UEs to which reception bandwidths of 10 MHz and 20 MHz are allocated in a 20-MHz system band.
Referring to FIG. 1, first, second and third UEs 100, 102 and 104 (UE1, UE2 and UE3) each having a minimum reception bandwidth of 10 MHz co-exist with a fourth UE 106 (UE4) having a minimum reception bandwidth of 20 MHz in the system with the 20-MHz bandwidth. In this case, a PCH signal is transmitted across the total 20-MHz system band, referred to herein as Method 1, or across different 10-MHz bands, referred to herein as Method 2.
Method 1 is not viable for UE1 because a first PCH signal (PCH1) is transmitted in the 20-MHz band wider than the reception band of UE1. Since the center Radio Frequency (RF) frequency 116 of UE1 is set in the left 10-MHz band, UE1 can receive a second PCH signal 110 (PCH2) in this band.
Similarly, for UE2, the center RF frequency 118 of UE2 is set in the left 10-MHz band and thus UE2 can receive a third PCH signal 112 (PCH3) in this band. As a result, the system has to transmit PCH2 and the PCH3 redundantly in the two 10-MHz bands in order to support UE1 and UE2 simultaneously, thereby increasing overhead.
The center RF frequency of UE3 is set to the center 114 of the 20-MHz band. Method 1 is not viable for UE3 because the transmission band of PCH1 is beyond the reception band of UE3. Even though UE3 receives PCH2 110 or PCH3 in Method 2, UE3 cannot carry out a paging procedure normally because the reception is possible only in part of each of the transmission bands of PCH2 and PCH3.
UE4 with 20 MHz as a minimum reception bandwidth can receive PCH1, PCH2 and PCH3 in Method 1 or Method 2 because the 20-MHz band covers the transmission bands of PCH1, PCH2 and PCH3.
Accordingly, there exists a need for developing a method for enabling reliable PCH reception in a UE with a bandwidth less than a 20-MHz system bandwidth when the UE receives a service in part of a system band in a bandwidth-scalable system.